Method For Encapsulating Semiconductor Dies

ABSTRACT

The present invention describes two methods ( 200, 400 ) for encapsulating semiconductor dies. Both methods ( 200, 400 ) involve attaching an encapsulation spacer ( 102, 302, 302   a,    302   b ) having one or more apertures ( 104, 304 ) on an associated substrate ( 150 ) so that a group of chips ( 160 ) is located within the aperture ( 104, 304 ). The first method ( 200 ) involves dispensing encapsulant ( 103 ) directly into an aperture. The second method ( 400 ) involves attaching an encapsulant delivery layer ( 350, 351 ) onto the encapsulation spacer and discharging encapsulant into an aperture via a recessed gate ( 308 ).

RELATED APPLICATIONS

A corresponding PCT patent application is filed on the same day as this case but it relates to the system for encapsulation of semiconductor dies.

FIELD OF INVENTION

The present invention relates to methods for encapsulation of semiconductor dies that does away with cavity moulds associated with injection or transfer molding. In particular, this invention relates to methods of discharging encapsulant into a cavity defined by an encapsulation spacer disposed on an associated substrate or carrier.

BACKGROUND

Conventional methods used in semiconductor die packaging involve the process of die bonding, wire bonding, encapsulation moulding, deflashing and singulation. Transfer moulding is typically used to encapsulate a group of semiconductor dies and the respective bonded wire interconnections with a conductive substrate to form a semiconductor package. In the process, the conductive substrate, with wire bonded dies, is placed in a lower mould plate of split-cavity. By clamping the upper mould plate onto the lower mould plate with a periphery of the substrate in between the split mould plates, injecting a liquefied encapsulant into the mold cavity, and allowing the encapsulant to cure, the dies are physically sealed and protected from the external environment. By singulating the plastic package, individual semiconductor chips are obtained.

Due to the use of high pressure in delivering the encapsulant, some of the bond wires may be dislodged or moved into contact with an adjacent bond wire. The other problem area is to design reservoirs, runners, gates and air vents to give encapsulant flow characteristics that are sufficient to meet void-free encapsulation. These moulds are expensive and require constant cleaning to remove the encapsulant from channels inside the moulds.

It can thus be seen that there exists a need for new systems and methods of encapsulating semiconductor dies by overcoming disadvantages of the existing prior art.

SUMMARY

The following presents a simplified summary to provide a basic understanding of the present invention. This summary is not an extensive overview of the invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalised form as a prelude to the detailed description that is to follow.

The present invention seeks a simple and cost effective system and method for encapsulating semiconductor dies by doing away with conventional cavity molds associated with injection or transfer moulding; in effect, the costs of making the toolings for the encapsulation spacer are lower than that for making the conventional cavity moulds. With the present invention, a small and simple press, such as a 4-pole press, with simple platen and pressure plate is sufficient for use with this invention. These tooling are generally simple and flat metal parts and obviate the need for constant cleaning, as in the case of cavity moulds, and this translate to higher productivity in the use of this invention.

In one embodiment, the present invention provides a method for encapsulating semiconductor dies. The method comprises: attaching an encapsulation spacer (102,302) on a substrate, on which are attached some semiconductor dies, such that a group of dies disposed according to conductive patterns on the substrate is seen through an aperture (104,304) on the encapsulation spacer (102,302); dispensing an encapsulant into the aperture; and allowing the encapsulant to cure.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which:

FIG. 1A illustrates an encapsulation system according to an embodiment of the present invention;

FIG. 1B illustrates an encapsulation spacer according to an embodiment of the encapsulation system shown in FIG. 1A;

FIGS. 2A-2G illustrate the various steps involved in using the encapsulation system shown in FIG. 1A; FIG. 3A illustrates an encapsulation system according to another embodiment of the present invention; FIG. 3B illustrates an encapsulation spacer according to an embodiment of the encapsulation system shown in FIG. 3A, whilst FIG. 3C illustrates an encapsulant delivery layer for use with the encapsulation spacer shown in FIG. 3B; FIG. 3D illustrates an encapsulation spacer according to another embodiment of the encapsulation system shown in FIG. 3A, whilst FIG. 3E illustrates an encapsulant delivery layer for use with the encapsulation spacer shown in FIG. 3D;

FIGS. 4A-4F illustrate the various steps in using the encapsulation system shown in FIG. 3A.

DETAILED DESCRIPTION

One or more specific and alternative embodiments of the present invention will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that this invention may be practised without such specific details. Some of the details may not be described at length so as not to obscure the invention. For ease of reference, common reference numerals or series of numerals will be used throughout the figures when referring to the same or similar features common to the figures.

FIG. 1A shows an encapsulation system 100 according to an embodiment of the present invention. As shown in FIG. 1A, the encapsulation system 100 is made up of an encapsulation spacer 102 being attached to a semiconductor substrate or carrier 150. One semiconductor die or chip 160 or more is/are attached to the substrate/carrier 150 according to a conductive pattern on the substrate/carrier. The die/chip 160 may be a wire-bonded device and the substrate 150 is an associated wiring board, such as a QFN leadframe, a flexible substrate, a ball grid substrate, etc. The encapsulation spacer 102 has a plurality of through apertures 104. FIG. 1A shows a simple encapsulation spacer 102 having one row of apertures 104 for ease of description, where a group of dies/chips 160 are located inside each aperture 104 when the encapsulation spacer 102 is attached to a substrate/carrier 150. Along one or more sides of each aperture 104, there is/are overflow reservoir(s) 110. Thickness of the aperture 104 is predetermined according to height of the die/chip 160 to be encapsulated and the amount of overfill on top of the die/chip. Each overflow reservoir 110 is connected to the respective aperture 104 by air vents 114. By filling the apertures 104 with an encapsulant 103, applying heat and pressure on the encapsulant so as to minimise any void space therein, allowing the encapsulant to cure and then singulating the encapsulated dies/chips into individual packages, the encapsulation system 100 provides a simple and cost effective method to form semiconductor packages.

The encapsulation spacer 102 need not be in the form of a panel as shown in FIG. 1A. In another embodiment, an encapsulation spacer is formed as an individual encapsulation ring 102. FIG. 1B shows the encapsulation ring 102 is formed in a quadrilateral shape, but it is not so limited in shape. As in the previous embodiment, the overflow reservoir 110 is connected to the inside of the encapsulation ring 102 by air vents 114.

In another embodiment of the encapsulation ring 102, there is an additional overflow reservoir 110 a. In one embodiment, the additional overflow reservoir 110 a is located opposite the overflow reservoir 110. In another embodiment, the additional overflow reservoir 110 a is round in shape and is located at a corner of the encapsulation ring that is opposite the overflow reservoir 110. In yet another embodiment, the encapsulation ring 102 has both types of such additional overflow reservoirs 110 a and associated air vents 114 a.

In one embodiment, the encapsulation spacer 102 is made of metal. In another embodiment, the encapsulation spacer is made of thermoplastic. The encapsulation spacer may be formed by conventional machining, moulding, etching, laser cutting or shaping methods. For example, the encapsulation spacer 102 may be made by etching on a metal piece, preferably from copper. In another example, the encapsulation spacer 102 may be made by masking a metal piece and building the exposed metal piece by plating it with a metal, such as copper. The material of the encapsulation spacer is not so limited; any other material that is low cost and easily formed by conventional machining or shaping may be used.

In FIGS. 1A and 1B, the encapsulation spacer/ring 102 is shown to consist of a single layer. In another embodiment, the encapsulation spacer/ring 102 a is made up of two or more layers, where adjacent layers may be joined by means of adhesive. The depths of the air vents 114,114 a and overflow reservoirs 110,110 a may be defined by the thickness of the relevant layer that make up the encapsulation spacer/ring 102 a. An advantage of this embodiment is that the layers to build the encapsulation spacer 102 are either plain solid or have the aperture 104; in this way, the height of the encapsulation spacer 102 is configurable according to the dies 160 to be encapsulated.

In use, the encapsulation spacer 102,102 a may be mounted on the substrate 150 by means of adhesive. FIGS. 2A-2G illustrate the process 200 of encapsulating semiconductor dies/chip using the above encapsulation spacer/ring 102,102 a. As shown in FIG. 2A, groups of dies/chips 160 are mounted 210 on the substrate 150 according to the conductive patterns on the substrate. In FIG. 2B, the encapsulation spacer/ring 102,102 a is mounted 220 on the substrate 150 by means of adhesive 118. The encapsulant 103 is then dispensed 230 into each aperture 104 of the encapsulation spacer 102,102 a or inside the encapsulation ring 102,102 a until the encapsulant 103 reaches the top of the encapsulation spacer/ring 102,102 a and is about to overflow into the overflow reservoir(s) 110,110 a via the respective air vents 114,114 a. Dispensing of the encapsulant may be carried out manually or automatically via a metering system. As shown in FIG. 2D, pressure may be applied 240 on the surface of the encapsulant after an aperture is filled. An overlay sheet 130 is then applied 250 over the top of the encapsulation spacer/ring 102,102 a to cover the encapsulant 103. The entire assembly is then disposed 260 inside a press where a platen, shaped and dimensioned according to the aperture 104 or inside of the encapsulation ring 102,102 a, applies 265 heat and pressure to the encapulant 103. The heat and pressure may be maintained for a predetermined period of time to allow the encapsulant 103 to cure, at least partially. FIG. 2G shows the dies 160 on the substrate 150 being encapsulated inside the aperture 104 of the encapsulation spacer 102,102 a at the end of the process 100. The entire assembly may then be disposed inside an oven to complete curing the encapsulant 103. After the encapsulant 103 is fully cured, the encapsulated dies/chips are singulated to form individual semiconductor packages.

FIG. 3A shows an encapsulation system 300 according to another embodiment of the present invention. The encapsulation system 300 is made up of the substrate/carrier 150, an encapsulation spacer 302 and an encapsulant delivery layer 350. As shown in FIG. 3A, the encapsulation spacer 302 is attached to the substrate/carrier 150 and the encapsulant delivery layer 350 is in turn attached to the encapsulation spacer 302; such attachments may be by means of adhesive 118. The present invention is clearer when individual parts of the encapsulation system 300 are described.

FIG. 3B shows the encapsulation spacer 302 according to an embodiment of the present invention. The encapsulation spacer 302 is exemplified as an elongate strip, which has a plurality of apertures 304. In FIG. 3B, the apertures 304 are aligned in a row along a longer dimension of the elongate strip for simpler description but they are not so limited. As in the previous embodiment, a group of semiconductor dies/chips 160 are attached to the substrate/carrier 150 such that the dies/chips are seen within an aperture 304 and a thickness of the encapsulation spacer 302 at the aperture defines a thickness of the encapsulant around the die/chip 160.

On the shorter dimension of the elongate strip, as shown in FIG. 3B, there are four reliefs 320. The reliefs 320 are dimensioned so that they provide finger and thumb gripping spaces, for example when the encapsulation spacer 302 is to be peeled off from the substrate/carrier 150 or when the encapsulant delivery layer 350 is to be peeled off from the encapsulation spacer 302 after the encapsulant has cured.

The right side of the encapsulation spacer 302 has a larger margin than the left hand side, as seen in FIG. 3B. In the right hand margin, the closed phantom line 306 shows the location of the encapsulant 103 stored in the encapsulant delivery layer 350 when the encapsulant delivery layer 350 is attached to the encapsulation spacer 302. A recessed gate 308, on the rear side of the encapsulation spacer 302 as seen in FIG. 3B, extends from inside the closed phantom line 306 to the associated aperture 304. A plane 309 defined by another phantom line cuts through the recessed gate 308. The area of the encapsulation spacer 302 on the right hand side of the plane 309 may be broken or sheared off after encapsulant is delivered into the apertures 304 and has at least partially cured. On the left and rear side of each aperture 304, as seen in FIG. 3B, is an overflow reservoir 310. An air vent 314 connects each overflow reservoir 310 to the respective aperture 304.

In one embodiment, the encapsulation spacer 302 is made of a unitary layer. For example, when the encapsulation spacer 302 is metallic, the built-up layer made be deposited by plating a metal on a substrate whilst depressions or apertures may be formed by masking and etching away the exposed metal surface. In another embodiment, the encapsulation spacer 302 a is made up of two or more layers; the adjacent layers may be joined by means of adhesive; in another example, the adjacent layers may be laminated together; the depths of the recessed gates 308, air vents 314 and overflow reservoirs 310 may be defined by the thicknesses of the relevant layers that make up the encapsulation spacer 302 a.

FIG. 3C shows an encapsulant delivery layer 350 according to an embodiment of the present invention for use with the encapsulation spacer 302,302 a. As shown in FIG. 3A, the encapsulant delivery layer 350 is dimensioned to match the encapsulation spacer 302,302 a, where the encapsulant is stored in a reservoir 352. The encapsulant delivery layer 350 is made up of a thin and flexible plastic but is strong and resilient enough to hold the encapsulant in the reservoir 352. In an example, the encapsulant delivery layer 350 may be made by conventional plastic moulding, such as injection or transfer moulding. Before use, the encapsulant delivery layer 350 may be covered by a peel-off layer; by removing the peel-off layer, an adhesive on the encapsulant delivery layer 350 is exposed and the encapsulant delivery layer 350 can then be attached onto the encapsulation spacer 302,302 a. In use, the encapsulation system 300 is placed inside a press and pressure on the reservoir 352 collapses the reservoir to deliver the encapsulant through the recessed gate 308 into the associated aperture 304 to encapsulate the dies/chips 160 disposed on the substrate/carrier 150. After the encapsulant around the dies/chips has cured, the encapsulant delivery layer 350 may be peeled off; alternatively, the encapsulation spacer 304,304 a together with the encapsulant delivery layer 350 may be broken or sheared at the plane 309.

FIG. 3D shows an encapsulation spacer 302 b according to another embodiment of the present invention. The encapsulation spacer 302 b is similar to the encapsulation spacer 302,302 a except that the recessed gate 308 starts with a recess 308 a. Each recess 308 a corresponds with a discrete encapsulant reservoir 353 on a matching encapsulant delivery layer 351 shown in FIG. 3E. In another embodiment, the area around the recess 308 a may be shaped and dimensioned to overlap the associated encapsulant reservoir 353, and the area around the recessed gate 308 is sufficient to adhere to the encapsulant delivery layer 351 to allow encapsulant to be delivered into the aperture 304, such that material around the recess 308 a and recessed gate 308 is redundant; this redundant material when removed forms openings 324.

FIGS. 4A-4F illustrate the process 400 of encapsulating semiconductor dies using the encapsulating spacer 302,302 a,302 b. As shown in FIG. 4A, groups of dies 160 are mounted 410 on the substrate 150 according to the conductive pattern on the substrate. In FIG. 4B, the encapsulation spacer 302,302 a,302 b is mounted 420 on the substrate, for example by adhesive. In FIG. 4C, the reservoir or pot 352,353 of an encapsulant delivery layer 350,351 is filled 430 with encapsulant 103. In FIG. 4D, the encapsulant delivery layer 350,351 is then attached to the encapsulation spacer 302,302 a,302 b. The entire assembly or system 300 is then disposed inside a press where a platen, shaped and dimensioned according to the aperture 304, applies 440 heat and pressure to the encapsulant 103. This is followed by collapsing 450 the reservoir or pot 352,353 of the encapsulant delivery layer 350,351, for example, by extending a ram on the reservoir/pot, as shown in FIG. 4E. The heat and pressure may be maintained for a predetermined period of time to allow the encapsulant 103 to cure, at least partially, as shown in FIG. 4F. After the encapsulant is cured and the assembly is removed from the press, the encapsulation spacer 302,302 a,302 b and encapsulant delivery layer 350,351 are broken or sheared off 460 along plane 309 before the encapsulated dies are singulated to form individual semiconductor packages. Alternatively, the encapsulant delivery layer 350,351 are removed prior to singulation to form individual semiconductor packages.

While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations thereof could be made to the present invention without departing from the scope of the invention. For example, the encapsulation spacer 102,102 a,302,302 a,302 b may have a vacuum channel 111,311 disposed alongside each overflow reservoir 110,110 a,310. Each vacuum channel 111,311 may have a vacuum port 312 for connection to a vacuum system as when necessary; a vacuum opening 362 corresponding to the vacuum port 312 may then be provided on the encapsulant delivery layer 350. A control gate 315 connects an overflow reservoir to the associated vacuum channel 311. Whilst a panel layout of the encapsulation system 300 has been described, the system 300 is also applicable for use with individual encapsulation rings and the encapsulant delivery layer 351. 

1. A method of encapsulating (200,400) semiconductor dies comprising: attaching an encapsulation spacer (102,302) on a substrate, on which are attached some semiconductor dies, such that a group of dies disposed according to conductive patterns on the substrate is seen through an aperture (I 04,304) on the encapsulation spacer (102,302); dispensing an encapsulant into the aperture; and allowing the encapsulant to cure.
 2. A method according to claim 1, wherein the aperture is configurable in shape and dimensions according to the conductive patterns on the substrate.
 3. A method according to claim 1, further comprises allowing the encapsulant to overflow from the aperture into an overflow reservoir via an air vent so that the aperture is tilled completely and void space is minimised.
 4. A method according to claim 1, wherein dispensing of the encapsulant is directly into the aperture and is carried manually or automatically via a metering system.
 5. A method according to claim 4, further comprises disposing an overlay layer over a top the encapsulant dispensed in the aperture.
 6. A method according to claim 1, wherein dispensing of the encapsulant is carried out by attaching an encapsulant delivery layer onto the encapsulation spacer and collapsing a reservoir/pot of encapsulant disposed in the encapsulant delivery layer.
 7. A method according to claim 6, wherein delivery of the encapsulant from the encapsulant delivery layer is via a recessed gate formed on the encapsulation spacer.
 8. A method according to claim 4, further comprises applying heat and pressure on the encapsulant disposed in the aperture.
 9. A method according to claim 8, further comprises maintaining the heat and pressure on the encapsulant for a predetermined period of time, at least to partially cure the encapsulant.
 10. A method according to claim 9, further comprises disposing the substrate, encapsulation spacer and encapsulant in an oven and allowing the encapsulant to fully cure.
 11. A method according to claim 10, further breaking of the encapsulation spacer and encapsulant delivery layer along the shear plane (309).
 12. A method according to claim 10, further comprises separating the encapsulation spacer from the substrate, separating the overlay layer from the encapsulation spacer and/or separating the encapsulant delivery layer from the encapsulation spacer.
 13. A method according to claim 12, wherein separating the encapsulation spacer from the substrate or separating the overlay layer and encapsulant delivery layer from the encapsulation spacer is made easier by providing reliefs on a periphery edge of the encapsulation spacer.
 14. A method according to claim 1, wherein the encapsulation spacer is of a unitary layer.
 15. A method according to claim 1, wherein the encapsulation spacer comprises two or more layers, and the adjacent layers are joined by means of adhesive.
 16. A method according to claim 1, wherein the encapsulation spacer further comprises a vacuum channel disposed along side an associated overflow reservoir, and the vacuum channel is connected to the associated overflow reservoir by a control gate.
 17. A method according to claim 16, wherein the depth of the aperture, recessed gate, overflow reservoir, air vent, vacuum channel or control gate is configurable according to thickness of the relevant layer of the encapsulation spacer.
 18. A method according to claim 6, further comprises applying heat and pressure on the encapsulant disposed in the aperture. 